In a gas turbine engine, air is pressurized in a compression module during operation. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generating engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
The compression system includes a rotor assembly comprising a plurality of rotor blades extending radially outward from a disk. More specifically, each rotor blade has a dovetail which engages with the disk, a platform forming a part of the flow path, and an airfoil extending radially from the platform to a tip. The platform may be made integral to the blade or, alternatively, made separately.
In some designs, the rotor blade, especially those in a fan rotor and the front stages of a multistage compression system, have a pair of circumferentially extending shrouds on the airfoil, one projecting from the pressure surface and one projecting from the suction surface. The shrouds are located at a radial location between the blade dovetail and the blade tip. In some other designs, the shrouds may be located at the tip of the blade airfoil. During normal operation of the compression system, the blades twist and the shrouds on adjacent blades contact with each other, forming a shroud ring that provides support to the blades. During engine operation, the shroud ring resists vibration and twisting of the blades. The term “midspan shroud” is used herein to refer to all supports on fan and compression system blades that contact with each other during operation, and includes all supports located anywhere on the span of the blade, including supports at the tip of the blade. The “midspan shrouds” as used herein, may be located anywhere along the blade span, not just at the midpoint of the span.
During certain abnormal events, such as a bird impact, other foreign object impact, or stalls during engine operation, the normal contact between the shrouds of adjacent blades is disturbed. The contact forces become high and misaligned due to the impacts and the shrouds become disengaged fully or partially. This is called “shingling” of the blades. Shingling causes significant wear and tear damage on the midspan shrouds. When the speed of the compressor rotor drops, the shingled blades may rebound, causing further wear and tear on the shrouds.
Fan or compressor blades sometimes have wear pads brazed on the contact faces on the midspan shrouds. Wear pads have been used on blades to address the wear problem. For example, some compressor blades contain a brazed-on WC—Co wear pad to reduce wear between two rubbing midspan shrouds.
The blades may comprise titanium or alloys thereof (i.e., Ti 6Al-4V and/or Ti 8Al-1V-1 Mo alloys) having a beta transus temperatures at or slightly above 1800° F. (about 982° C.). The wear pads are conventionally brazed to the titanium blade using a titanium-copper-nickel (TiCuNi) alloy braze foils. Diffusion occurs between TiCuNi braze foil and WC—Co wear pad during high temperature braze. Titanium forms brittle compounds with the alloying elements of the wear pad in the braze joint. As a result, the braze joint provides a high hardness (about 1200 KHN) W—Co—Ti—Cu—Ni alloy. The braze interface exhibits cracking at impact energies as low as 0.30 joules, and the wear pad may be liberated from the substrate at the brittle braze interface at an impact energy of 0.60 Joules.
Industrially available braze alloys have been unable to meet a demand for low braze temperature (i.e., below 1800° F.), while providing the high ductility and low cost necessary for aircraft engine applications. For example, Nioro (Au 82% and Ni 18%) and Nicoro80 (Au 81.5%, Cu 16.5% and Ni 2%) are heavy in gold and light in copper and therefore are expensive and have poor wetting properties and ductility. Alloys incorporating Au 35%, Cu 62% and Ni 3% have liquidus temperatures at or above 1886° F. (about 1030° C.), which is not acceptable for brazing WC—Co substrates to titanium alloys.
Some known brazing alloys incorporating silver have also failed to meet the combined demands of low braze temperatures, high ductility and low cost necessary for aircraft engine applications. For example, CUSIL™ (63.3Ag-35.1Cu-1.0Ti) alloy lacks nickel and may cause wettability problems with WC—Co if braze times are short. Another silver alloy, 95% Ag-5% Al, lacks both copper and nickel and has been unsuccessful in corrosion wear applications of WC—Co on Ti-6Al-4V. A third alloy, a non-silver containing softer braze alloy of high copper content, Copper-ABA® (Cu-2% Al-3% Si-2.25% Ti) as well as a 50% Au-50% Ag and 69% Au-25% Ag-6% Pt have braze temperatures at or above the beta transus temperature of Ti-6Al-4V and therefore cannot be used.
Accordingly, there is a need for high ductility, impact resistant brazing alloys with brazing temperatures below the beta transus temperature of the substrate titanium alloy. In particular, there is a need for brazing alloys for brazing WC—Co materials to titanium alloys without forming a brittle braze interface.